SELECT   city,
         MAX(DATEDIFF(year, dbo.Employees.BirthDate, GETUTCDATE ()) ) 'greatest age', 
         MIN(DATEDIFF(year, dbo.Employees.BirthDate, GETUTCDATE ())) 'smallest age',
         AVG( DATEDIFF(year, dbo.Employees.BirthDate, GETUTCDATE ())) 'average age'
FROM     [Northwind].[dbo].[Employees] 
group by City